A backup appliance, such as the Data Domain Restorer (DDR) made by EMC Dell, allows data to be written to de-duplicated storage from different backup clients using different protocols such as Network File System (NFS), Common Internet File System (CIFS), Data Domain Boost over Fibre Channel (DFC), Virtual Tape Library (VTL), or Virtual Disk (VDISK). The primary data ingest mechanisms into backup appliance are Ethernet and Fibre Channel. The backup appliance can provide de-duplication for the customer across the different backups, so it is essential for the customer to be able to do backups using different protocols to the same backup appliance to cater to different backup use cases.
In a conventional datacenter environment, different backup clients are managed by different administrators and they choose to configure the backup appliance independently for their needs. These backup clients could use different protocols or different sets of devices. The conventional SCSI target subsystem uses a global locking scheme for configuring Small Computer System Interface (SCSI) devices. This means that configuration of SCSI devices can affect input/output (I/O) of other SCSI devices and can even cause the other SCSI devices to be blocked, disrupting I/O access to other SCSI devices. In other words, conventional SCSI targets allow the devices to be managed as one group, so only one client can manage the devices required by that client even if it is configuring only a subset of devices. The restriction that only one client is allowed to configure and manage the devices at a time leads to increased maintenance windows. In other words, this may negatively impact deployment and configuration time in an enterprise environment.
In addition, in certain scenarios the backup appliance can be slowed down by events such as garbage collection, low free storage space, slow disk, etc. As a result, the devices can respond slowly or become unresponsive leading the backup clients to reset the devices. For example, a backup WRITE could be slow because the backup appliance is nearly full, leading to device resets, but another backup client only performing READs will also get affected. The device reset is a heavy-handed process requiring total control of the devices. In certain scenarios if a subset of devices are performing slowly, a target reset from a backup could affect all the devices on the target, leading to failed backups on all the clients, which can become unacceptable in enterprise datacenters requiring 24/7 availability.
Furthermore, different protocols have different performance requirements, but conventional SCSI targets provide similar performance to all the protocols. Customers expect similar performance from all the devices, but there could be scenarios where some devices starve other devices, causing them to respond slowly and resulting in failed backups. For example, DFC, which utilizes source deduplication, has very little traffic on the wire compared to VDISK. As a result, a fast DFC client can consume all the resources on the target and starve the VDISK devices, leading to backup failures.
Moreover, the device recovery timeouts vary vastly among protocols: VDISK and DFC clients are very sensitive to latency compared to VTL clients. So slow VTL can wreak havoc on backups on VDISK and DFC clients. The nature of the protocols also varies: VTL is a sequential protocol requiring strict adherence to ordering of commands, unlike VDISK and DFC protocols. The protocols also have different performance needs requiring different amounts of resources that cannot be provided by the conventional SCSI target.
The problem is aggravated in the Software Defined Data Centers (SDDCs) where the configuration and management is driven by applications and not handled manually by the users. The applications would like to automate the backup workflows in parallel for different use cases, but with the conventional SCSI target, all requests would get serialized at the backup appliance, leading to reduced productivity for the clients. Clients of backup appliances may not be able to realize the full benefits of the automation provided in the SDDC environment when using a conventional SCSI target.